1. Field of the Invention
The present invention relates to an atomic frequency standard and, more particularly, to a rubidium- or cesium-using atomic frequency standard, which frequency standard utilizes a laser-pumped optical-microwave double resonance.
2. Description of the Related Art
Today, a rubidium (.sup.87 Rb) or cesium (.sup.133 Cs) atomic frequency standard is used as a small, practical high-performance secondary frequency standard in multifarious fields, such as communications, broadcasting, navigation and GPS (Global Positioning System) satellites.
The present inventor and some other have proposed to use a semiconductor laser as a pumping light source of a Rb atomic frequency standard in place of a Rb lamp to improve the performance, and in particular, to effectively use the modulation transfer effect between microwave and laser light to provide short-period frequency stabilization, as disclosed in "Experiments On A Semiconductor Laser Pumped Rb Atomic Clock," by M. Hashimoto and M. Ohtsu, IEEE J. Quantum Electron. QE-23, 446-451 (1987).
This semiconductor laser pumped Rb atomic frequency standard will be briefly described referring to the block structure shown in FIG. 3.
The atomic resonance section of the frequency standard comprises a semiconductor laser 1, a resonance cell 2 having Rb gas and buffer gas sealed in a gas cell, and a microwave cavity resonator 3 disposed surrounding the whole resonance cell 2. A signal of a predetermined frequency which is the cause of a microwave generated from a voltage-controlled crystal oscillator 5, is multiplied by a given factor in a frequency synthesizer 6 and is subjected to phase modulation by a wave with a modulation angular frequency .omega..sub.m from a modulation oscillator 7. The resultant signal (e.g., 6.83 GHz in case of Rb) is applied to the microwave cavity resonator 3. Light coming from the semiconductor laser 1 led via an attenuator 1a to the resonance cell 2. Thereby, the light frequency is modulated by the modulation transfer and is absorbed by an optical-microwave double resonance. The light transmitted from the resonance cell is subjected to photoelectric conversion by a photosensor 4.
A signal from the photosensor 4 is supplied via an amplifier A to a phase sensitive detector (PSD) 8 where it is subjected to PSD using a reference signal of a modulation angular frequency .omega..sub.m from the modulation oscillator 7. The output of the PSD 8 is supplied via a low-pass filter (LPF) 9 to the voltage-controlled crystal oscillator (VCXO) 5.
Reference numeral "10" denotes a current source for driving the laser 1.
FIG. 4 illustrates the energy level of an .sup.87 Rb atom. In this diagram, a model of the .sup.87 Rb-D.sub.2 line is illustrated by a 3-level atomic system for simplicity.
A Rb atomic oscillator shown in FIG. 3 is an automatic control system, which stabilizes the output frequency of the voltage-controlled crystal oscillator 5 based on the microwave resonance between the hyperfine levels of the .sup.87 Rb atom in the ground state.
An optical-microwave double resonance method is used as a technique to detect the microwave resonance. The optical-microwave double resonance is caused by providing a population difference between the hyperfine levels in the ground state in FIG. 4 presenting the energy level illustration of the .sup.87 Rb atom by means of laser pumping, and causing the interaction between the atoms and the microwave obtained by multiply the reference frequency of the microwave from the voltage-controlled crystal oscillator 5 by a given factor.
That is to say, referring to FIG. 4, the ground state 5S is divided into two hyperfine levels: F=1 and F=2. Upon absorption of the light from the semiconductor laser 1, .sup.87 Rb is pumped to the excited level, 5P, from the level of F=1, and is transited to the levels of F=1 and F=2 with an equal probability. .sup.87 Rb transited to the level of F=1 is pumped again to the excited level 5P by the light from the laser 1. In this manner, .sup.87 Rb at the level of F=1 gradually decreases, thus reducing the amount of light absorbed by the resonance cell 2. Under this situation, when an electromagnetic wave of approximately 6.8 GHz, which corresponds to the energy difference between F=1 and F=2, is applied to the cavity resonator 3, .sup.87 Rb at the level of F=2 transits to F=1 by the stimulated emission, and absorption by the resonance cell 2 is again increased. The light passing the resonance cell 2 is detected by the photosensor 4 and the voltage-controlled crystal oscillator 5 is controlled in such a manner as to minimize the amount of this transmitted light or maximize the absorbed light. Since the output frequency of the oscillator 5 is determined by the hyperfine levels of .sup.87 Rb, it is well stabilized. For instance, with the multiplication factor of the frequency synthesizer 6 being about 1360, a frequency of 5 MHz would be acquired.
Although the intensity of the pumping laser beam passing the gas cell 2 having .sup.87 Rb sealed therein is detected by the photosensor 4, a very slight absorption spectrum is acquired by occurrence of the optical-microwave double resonance. This transmission light signal is amplified by the amplifier A, then supplied to the PSD 8 where it is subjected to PSD by the modulation wave having the modulation angular frequency .omega..sub.m which is applied to the microwave. The resultant PSD output V.sub.PSD is used as an signal which is fed back via the low-pass filter 9 to the voltage-controlled crystal oscillator 5.
In analyzing under the modulation transfer between microwave and light, the PSD output V.sub.PSD is obtained as a signal similar to the FM side band spectrum. With the reference signal for PSD being cos(.omega..sub.m t-.theta.), the PSD output V.sub.PSD is expressed as follows: EQU V.sub.PSD =V.sub.0 [(B/2)cos.theta.+(C/2)sin.theta.] (1)
In the equation, B and C are given by ##EQU1## where V.sub.0 is a proportional constant, J.sub.q is a Bessel function of the q-th order, .delta. and .phi. are respectively the amount of amplitude attenuation and the amount of phase shift, which the electric field of light passing the gas cell 2 experience, and the subscription "q" (q=.+-.1, .+-.2, .+-.3), indicated that .delta. and .phi. are shifted by q.omega..sub.m from the microwave resonance angular frequency .omega..sub.M.
FIG. 5 shows the dependences of B and C spectrums on the microwave frequency in the equations 2 and 3.
From these diagrams, it should be understood that the B and C spectrums are both odd functions having zero at the center of the microwave resonance frequency.
Therefore, the PSD output V.sub.PSD is also an odd function as exemplified in FIG. 6, in accordance with the equation 1, and likewise takes zero at the center of the microwave resonance frequency. Accordingly, the output frequency of this atomic frequency standard can be fixed to the zero-crossing point (between A and B in FIG. 6) by feeding the PSD output V.sub.PSD back to the voltage-controlled crystal oscillator 5 via the low-pass filter 9 as shown in FIG. 3.
The above is the principle of stabilizing the frequency of a semiconductor laser pumped Rb atomic frequency standard for a short period of time using a technique of the modulation transfer between microwave and light (hereafter the control technique applied the FM side band spectroscopy) proposed by the present inventor et al. and this apparatus can provide a higher stabilization in the order of about one digit, as compared with the conventional atomic oscillator using a Rb lamp.
Even with the use of such a semiconductor laser pumped Rb atomic frequency standard, the microwave resonance frequency of .sup.87 Rb that is the reference frequency is shifted by an optical Stark effect produced by the electric field of the pumping light (hereafter the light shift) when the frequency of the pumping light varies.
FIG. 7 illustrates this event in which a change occurs in the spectrum shown in FIG. 5.
FIG. 8 also exemplifies deformations of the C spectrum.
In FIG. 7, the curve A is for the laser power density of 1008 .mu.W/cm.sup.2, the curve B for 360 .mu.W/cm.sup.2 and the curve C for 144 .mu.W/cm.sup.2, whereas in FIG. 8, the curve A is for the laser frequency offset of 1200 MHz, the curve B for 0 Hz and the curve C for -1200 MHz.
The light shift hinders the long-period frequency stabilization of the semiconductor laser pumped Rb atomic frequency standard.
The atomic frequency standard utilizing the optical-microwave double resonance pumped by a semiconductor laser still has a shortcoming that the short-period frequency stabilization of the atomic frequency standard is hindered by FM noise of light caused by the semiconductor laser section per se, which cannot be overcome the aforementioned well-known the control technique applied FM side band spectroscopy.
Even if the restriction of the long-period frequency stabilization based on the light shift and the hindrance short-period frequency stabilization originated from FM noise of light generated by the laser section are overcome, collision of the buffer gas and rubidium gas sealed in the gas cell 2 shifts the microwave resonance frequency, thus impairing the accuracy of the output frequency of the voltage-controlled crystal oscillator 5.
The above-described situations also apply to when using .sup.133 Cs substituting .sup.87 Rb.